Those ads, and the big money Mr. Romney has at his disposal to throw into them, were a big help to Romney in putting Newt Gingrich into the rear-view mirror when Mr. Gingrich seemed to be gaining on him.

Gingrich offered plenty for Romney and his backers to latch onto. There were ethics lapses in Congress (which seemed particularly to resonate with some voters); videos linking him to Nancy Pelosi, global-warming legislation, and other more liberal issues; and Gingrich's spotty personal life.

At first glance, there's a lot to mine from Santorum's past, too. There are the segments from his book about "radical feminists," seeming to disparage women in the workplace, and his comments criticizing contraception. He recently attacked the idea of prenatal testing, and he once compared homosexuality with "man on dog" sex. (The latter earned a now-famous reply from his Associated Press interviewer, who declared, "I'm sorry, I didn't think I was going to talk about 'man on dog' with a United States senator; it's sort of freaking me out.")

The trouble is, those statements, which may be useful in the general election, are almost impossible to use against him now. Santorum's base is the conservative wing of the Republican Party, particularly social conservatives. Romney, who is trying to persuade those voters that he's a better choice, can't risk directly mocking Santorum for some of his more out-there statements on conservative issues. Any such attacks might backfire, leading conservatives who are already unsure about Romney's conservative credentials to definitively conclude that he is, in fact, too moderate.

Instead, Romney is relying on an old standby: criticizing Santorum for not being fiscally conservative enough – voting to raise the national debt ceiling and supporting "earmarks" (pet projects for the home state) when he was in Congress.

One Romney TV ad currently airing in Michigan starts with an image of a man falling through water, as a voiceover intones, "America is drowning in national debt."

"Yet," the voice continues, "Rick Santorum supported billions in earmarks." It then cuts to an old video of Santorum on a talk show, telling the interviewer, "I have had a lot of earmarks. In fact, I'm very proud of all the earmarks I've put in bills."

Compared with Gingrich's messy divorces and ethics charges, or Herman Cain's women troubles, or Rick Perry's multiple insert-foot statements and debate missteps, earmarks and the debt ceiling may seem run-of-the-mill. Besides, is there anyone in Congress who hasn't voted for district earmarks at some point?

Romney is also working to paint Santorum as a Washington insider. "When Republicans go to Washington and spend like Democrats, you’re going to have a lot of spending,” Mr. Romney told a crowd in Ohio on Monday. “And that’s what we’ve seen over the last several years.”

And one current ad by the pro-Romney super PAC, Restore Our Future, ends with the statement: “Rick Santorum. Big spender. Washington insider.”

Still, the Romney line of attack appears to be making little headway. Part of the problem may simply come down to likeability. Whereas Romney compared fairly well on that front with Gingrich, new polling suggests that Romney's own favorability ratings are suffering, whereas Santorum is doing well.

Expect more negative ads and statements attacking Santorum in coming weeks, especially if Romney feels desperate. But finding a line of attack that really sticks may prove to be tricky.